Touchdown

About a minute and a half after the high gate, when the spacecraft reached about 500 feet, it passed low gate and entered the final landing phase about a minute from touchdown. This phase was designed ''to provide continued visual assessment of the landing site and to provide compatibility for pilot takeover from the automatic control.''36 The commander might wish to spend some time making decisions, so extra fuel was put in the delta v budget for the pilot to hover and inspect the landing site before actually descending to the surface. Different pilots might do things differently, so extra fuel was added in the budget for variations in pilot skills and techniques.37

Nobody really expected the pilots to land under control of the LPD (Bennett thought the fully automatic feature might be used on some future unmanned version of the LM). Any time after pitch over, the pilot could flick a switch from PNGS AUTO to ATTITUDE-HOLD, commanding the computer into yet another program, P66, or the computer would enter P66 automatically just before low gate. In P66, ''the astronaut has a choice of four modes of control, differing in the division of duties between astronaut and guidance computer.''38 Here the computer controlled the rate of descent, but the commander could change that rate in real-time. Using a ''rate of descent switch'' near his left hand, one click down increased or one click up decreased the descent rate by one foot per second. Also in P66 the commander could exercise ''rate control'' of attitude, so he could fly the vehicle like a helicopter while the computer managed his descent rate. This mode ''is manual, but is aided by automatic control loops, that is, the pilot has taken over direct control but he has stabilization loops to provide favorable responses.'' The earlier the commander entered P66 the further he could fly—at 1,000 feet he could fly for three minutes and head down range nearly three miles from the designated spot;taking over at 300 feet, he could travel only one-half mile in just over two minutes. P66 also had a true manual descent mode where the lefthand controller would throttle the descent engine directly and the righthand controller would control the RCS thrusters (very fuel-inefficient and difficult to fly, this mode was never used).

Now, while descending from 100 feet in final approach, the commander would ''null'' the lateral velocities—that is, stop the spacecraft's lateral motion. This would have been an easy enough task for the computer, but an automatic mode to slowly lower the LM in a hover was never used. Instead the commanders eye-balled the velocity needles, strained to see the lunar surface through the dust, and worked the stick to stop the motion while decreasing descent rate to about five feet per second. About 50 feet above the surface the commander slowed the descent to a touchdown rate of 3.5 feet per second. At this point the computer was precisely controlling the descent rate, and also automatically holding an attitude commanded by the pilot. Though these final control modes were often referred to as manual, they were really semiautomatic. Every LM commander landed with a significant degree of computer-aided control.

Below 100 feet, the commander could no longer abort by shedding the descent stage; he was committed to at least a momentary landing. From this point, he could land visually, based on what he saw out the window; or if his vision was obscured, land by looking at the instruments, primarily the altitude and velocity indicated by the PNGS.

Apollo missions landed in both visual and instrument modes, depending on the degree of dust they kicked up.

The final touchdown also presented delicate problems. Shutting off the engine too early would drop the LM to the surface with a crunch, possibly a splat. Shutting it off too late, however, could cause the engine to kick up too much dust and obscure the pilot's visibility. Worse, if the engine bell hit the surface and clogged, it could explode. Ideally, the commander shut the engine off at the moment of touchdown. The landing radar likely would not be reliable at these low altitudes, so an additional means was required to give a discrete indication of the shutdown time, at about five and a half feet above the surface.

Three of the LM's four legs had landing probes attached; these were long tubes with switches on the end that gave a positive indication of touchdown. When the probes touched down, a blue light in the LM cockpit illuminated, with the words ''lunar contact,'' prompting the astronauts to manually shut down the engine. The length of the probes reflected the estimated delay of the pilot's response, the amount of time it took to close the valves in the descent engine, and the amount of time it took to actually reduce the thrust to zero. The pilot could take up to one second to respond and still allow the engine complete shutdown at touchdown.

He would then enter P68 to confirm to the computer that the vehicle had landed. He also momentarily jogged the stick to ask the computer to hold a new attitude. If the LM landed on a slight slope, for example, and the automatic control system was still trying to keep the LM level, it would be thrusting away with the reaction jets to level the vehicle. P68 turned off all the control loops and commanded the computer to display its estimate of the latitude and longitude of the landing site. The LMP would also disable the AGS by entering a command in its memory address number 413.